1. Field of the Invention
The present invention relates to a reflecting plate used together with an optical position detecting device and a reflecting frame including the reflecting plate on a reflecting surface thereof.
2. Background Art
In recent years, an interactive white board (IWB) has been increasingly adopted. The IWB can display, on a screen thereof, results similar to drawing of characters or the like on a blackboard with chalk using a finger, an electronic pen, or other means as an input object. In addition, the IWB can also detect an operation input using the input object as a mouse input.
The IWB is formed of the combination of a display device and a coordinate input device. Examples of the input object include a finger, a stylus pen, and an electronic pen. Examples of the display device include a plasma display panel (PDP), a liquid crystal display (LCD), and a projector. Examples of the coordinate input device include position detecting devices of a tablet type, a touch panel type, an electromagnetic induction type, an ultrasonic type, and an optical type. Note that, in any coordinate input devices, an arithmetic device such as a computer is used to calculate coordinates of the input object.
The present specification focuses attention on a coordinate input device that adopts an optical position detecting technique in which an image of the input object is picked up by an image sensor. The number of adopted coordinate input devices including the image sensor is increasing year after year due to an excellent drawing responsiveness and a strong resistance to external noise such as infrared rays, solar rays, and a change in temperature.
JP Patent Nos. 3931030 and 3986710 are exemplified as patents relating to an interactive white board that adopts the coordinate input device including the image sensor. The coordinate input device of this type is formed of the combination of a position detecting device and a retro-reflecting plate.
The position detecting device of this type includes an image sensor, a light source unit, and a control board (arithmetic device). The image sensor is attached to a periphery of a position detecting surface. If two or more image sensors are provided, an image of the input object on the position detecting surface can be picked up at a plurality of points of view. The control board receives the picked-up image from the plurality of image sensors, and calculates the position of the input object according to the principle of triangulation.
The light source unit is formed of a group of a plurality of light sources such as LEDs, and is placed in the vicinity of each image sensor. Each LED emits a light ray to retro-reflecting plates that are placed so as to surround the position detecting surface. Note that, a reflecting tape having a surface on which a retro-reflective minute structure such as a prism structure is placed is generally used as the retro-reflecting plate. The light ray emitted from the LED is reflected on the retro-reflecting tape back towards the LED that is a light source. The image sensor receives this reflected light. That is, an image of the total length of the reflecting tape is picked up. Note that, when the input object approaches the position detecting surface, the input object blocks the emitted light. The portion of this blocked light is detected as a shade on the picked-up image.
The control board (arithmetic device) calculates the position of the input object on the position detecting surface on the basis of the position of the shade appearing on a plurality of the picked-up images.
Incidentally, in a conventional position detecting device, a lens is generally placed on a light receiving surface thereof in order to enable the total length of the reflecting tape to fall within the image-pickup range of the image sensor. This is because the image-pickup angle of each light receiving element constituting the image sensor is as small as several millimeters to ten-odd millimeters. Note that, the lens needs to have a size suited to such a small light receiving element. In the past, a wide-angle lens having a small size did not exist. For this reason, a lens having an angle of view of approximately 90 degrees, which is easy to obtain, has been used in many cases as the image-pickup lens for the position detecting device.
Accordingly, the image sensors need to be placed at both right and left ends of the detecting surface. In actuality, the image sensor placed at the upper left corner can pick up images of the reflecting tape surfaces placed at the right side and the lower side of the detecting surface, but cannot pick up an image of the reflecting tape surface placed at the left side because the reflecting tape surface placed at the left side is outside of the angle of view. Similarly, the image sensor placed at the upper right corner can pick up images of the reflecting tape surfaces placed at the left side and the lower side of the detecting surface, but cannot pick up an image of the reflecting tape surface placed at the right side because the reflecting tape surface placed at the right side is outside of the angle of view.
Despite that, in the conventional device, as the area of the detecting surface is larger, a distance between the image sensor placed at the upper left corner and the image sensor placed at the upper right corner is larger. Accordingly, the casing size of the position detecting device (including two image sensors, two light source units, and a control board (arithmetic device)) is increased unfavorably. In addition, it is necessary to specially prepare the position detecting device in accordance with the size of the detecting surface.
In order to solve this technical problem, the applicant(assignee) of the present application have proposed a position detecting device not depending on the size of a position detecting surface (JP Patent Application No. 2010-125813 A). Specifically, the applicant(assignee) of the present application have proposed a position detecting device having the horizontal length that can be shorter than the horizontal size of the position detecting surface.
The achievement of this position detecting device enables one position detecting device to be used in a coordinate input device having an arbitrary detecting surface size. Meanwhile, in a prism structure (a structure including a trihedral prism element) constituting a reflecting tape surface, conditions of the incident angle that allows light to be retro-reflected is relatively strict unfortunately. Particularly near lower ends on the respective right and left sides of the position detecting surface, the incident angle of illumination light may exceed a range that allows the light to be retro-reflected.